Unless you think Stanley Ann was not his mom, he would be a US citizen regardless of where he was born and who his father was. Though arguably not NBC.
Those birthers who want to claim Obama wasn't his "real" father just damage their own case, which is based on Pop not being a US citizen. If Pop is somebody else, there's a very good chance he was a citizen.
If Mom was enough of a party girl, there's a good chance she would not know who the bio-dad was, though he was pretty clearly of African ancestry and there weren't a whole bunch of those in HI at the time. Or now, FTM.